how to
photograph
wild birds

One-Shot AF is generally best suited for non-moving subjects, such as stationary objects or landscapes and portraits. Once the focus is set on an AF Point, the focus will not drift to other objects (i.e., the focus is locked). Therefore, One-Shot AF is preferable when you want to adjust the composition after getting the subject in focus.
The center AF Point is the most precise AF Point on almost all digital cameras, and One-Shot AF focuses faster than AI Servo AF, particularly in dark locations. The drawback is that a moving subject will result in blurred photos since the focus will not change once it is set.

Since centered compositions often look stale, I used One-Shot AF for this shot to first focus on the Grey Heron and then I adjusted the composition and captured the instant it seized a fish. Each time the Grey Heron moved, I would refocus and wait with the focus locked. The moment it caught the fish, I fired a burst of continuous shots.

AI Servo AF tracks and keeps focusing on a moving subject using motion prediction. Once the subject has been selected, the camera will keep it in focus as long as it stays within the AF frame. This mode is perfect for photographing birds soaring through the sky.
Because the human eye can follow horizontal motion fairly well, a seasoned photographer can keep a horizontally moving subject in focus using manual focus. But the eye is very poor at recognizing subjects moving toward the camera. In such cases, we can take in-focus photos by relying on the focus tracking of the AI Servo AF mode. (For example, AI Servo AF will keep focusing on a F1 racecar as it zooms toward you at tremendous speed along a circuit.)

Two Peregrine Falcon siblings scramble over some food provided by their parent. Their inexperienced flying is what probably makes them look as if they are playing. With AI Servo AF, we can focus on movements as complex as those of these two birds.